Here are other threads similar to this. Maybe you can find some answers there. Hatzlacha!
http://www.theyeshivaworld.com/coffeeroom/topic/help-with-biting-nails
http://www.theyeshivaworld.com/coffeeroom/topic/help-stop-knuckle-cracking
http://www.theyeshivaworld.com/coffeeroom/topic/am-i-going-to-gehenim
If you do find something, please let us know.